1. Field of the Invention
This invention relates to a low radiation, photocurable and thermosetting resin composition which is advantageously used as a solder resist to be used in the manufacture of rigid printed circuit boards, flexible printed circuit boards, and LSI packages such as BGA (ball grid array), CSP (chip size package) and tape carrier packages. More particularly, this invention relates to an alkali-developable, photocurable and thermosetting resin composition capable of forming a cured film which emits alpha rays which affect semiconductor elements in a fully low dose and excels in such properties as heat resistance, adhesiveness, whethering resistance, resistance to electroless plating, electrical properties, and resistance to PCT (pressure cooker test) required of the solder resist for IC packages, its cured film and a printed circuit board having a solder resist film formed from the cured film.
2. Prior Art
In general, as a solder resist for the industrial grade printed circuit boards, a liquid developing type solder resist which is irradiated with ultraviolet light and then developed to form an image and thereafter finally cured (finish curing) by heating and exposure to light is adopted from the viewpoint of ensuring highly accurate formation of circuits with high density. Further, with due respect to the problem of environmental safety, the liquid solder resist of the alkali developing type which implements development with a dilute aqueous alkali solution as a developer has come to play the leading role.
Furthermore, in view of high densification of the printed circuit board to cope with the recent trend of electronic devices toward decreasing weight and size, the IC packages called BGA (ball grid array), CSP (chip scale package), etc. are recently used instead of the IC packages called QFP (quad flat-pack package), SOP (small outline package), etc. These new packages have such structure that metals such as ball-like solder are arranged in an area on one side of a printed circuit board having a solder resist applied thereto, the semiconductor elements are directly connected thereto on the other side by wire bonding or through the medium of the bumps etc., and they are sealed by a sealing resin. As the solder resists which can be advantageously used for these packages, the compositions as disclosed in published Japanese Patent Applications, JP 11-315107,A and JP 11-288091,A may be cited.
In the above-mentioned resin composition for sealing the semiconductor, a general-purpose silica powder was conventionally used as an inorganic filler. However, such a silica powder is the crystalline silica obtained only by grinding a natural silica powder or molten silica (amorphous silica) obtained by grinding the natural silica powder after melting. Since these silica materials contain radioactive materials, such as uranium and thorium, they pose the problem that a semiconductor element is operated incorrectly due to alpha rays emitted therefrom. Particularly in the recent semiconductor memory elements having a remarkably increased degree of accumulation, this incorrect operation will cause a serious problem.
Moreover, in the solder resist to be used for a printed circuit board, inorganic fillers such as barium sulfate, barium titanate, powdered silicon oxide, amorphous silica, talc, clay, calcined kaolin, magnesium carbonate, calcium carbonate, aluminum oxide, aluminum hydroxide, and mica powder are used for the purpose of relaxing strain or imparting printability thereto. These fillers are obtained only by grinding the natural ore and subjecting the resultant powder to a chemical treatment or heat treatment and have not undergone the high purifying processing to remove the radioactive materials therefrom. Accordingly, it is expected that the semiconductor elements will be incorrectly operated due to alpha rays emitted from the inorganic filler in the solder resist.
Under such circumstances, in order to prevent incorrect operation of semiconductor elements, the solder resist which emit alpha rays in a lower dose is strongly desired as the solder resist to be used for LSI packages such as BGA (ball grid array), CSP (chip size package), and TCP (tape career package).
An object of the present invention, therefore, is to provide an alkali-developable, photocurable and thermosetting resin composition capable of forming a cured film which emits alpha rays which affect semiconductor elements in a fully low dose and excels in such properties as heat resistance, adhesiveness, whethering resistance, resistance to electroless plating, electrical properties, and PCT resistance required of the solder resist for IC packages, its cured film and a printed circuit board having a solder resist film formed from the cured film.
To accomplish the object mentioned above, the present invention provides an alkali-developable, photocurable and thermosetting resin composition characterized by comprising (A) a high purity synthetic silica powder having a uranium content of not more than 1.0 ppb (parts per billion) in place of the conventionally used natural inorganic filler, and further (B) a photosensitive prepolymer containing a carboxylic group and at least two ethylenically unsaturated groups in its molecule and having an acid value of solid content of 50 to 150 mg KOH/g, (C) a photosensitive (meth)acrylate compound, (D) a photopolymerization initiator, (E) an epoxy resin, and (F) a curing catalyst in combination. The term xe2x80x9c(meth)acrylatexe2x80x9d as used in this specification means acrylate, methacrylate, or a mixture thereof. This holds good for the similar expression.
In accordance with another aspect of the present invention, there is provided a cured film obtained by a method which comprises a step of forming a resist pattern on a substrate by forming a film of the photocurable and thermosetting resin composition mentioned above on the substrate and subjecting the film to exposure to light and development and a step of curing the film of the resultant resist pattern. The formation of the film on the substrate is performed by applying the photocurable and thermosetting resin composition mentioned above to the substrate and drying it or by laminating a dry film of the photocurable and thermosetting resin composition mentioned above on the substrate. The step of curing the coating film is carried out by subjecting to irradiation with active energy rays and subsequent thermal curing, or to thermal curing and subsequent irradiation with active energy rays, or to thermal curing only.
In accordance with still another aspect of the present invention, there is provided a printed circuit board having a solder resist film formed on a circuit board provided with a conductor layer of a prescribed circuit pattern, characterized in that the solder resist film mentioned above is formed from a cured film of the photocurable and thermosetting resin composition mentioned above. The cured film is obtained by subjecting the photocurable and thermosetting resin composition mentioned above to irradiation with active energy rays and/or thermal curing.
Since the cured film formed from the photocurable and thermosetting resin composition of the present invention emits alpha rays which affect semiconductor elements in a fully low dose and excels in such properties as heat resistance, adhesiveness, whethering resistance, resistance to electroless plating, electrical properties, and PCT resistance required of the solder resist for IC packages, it is possible to manufacture the highly reliable LSI packages which will not be incorrectly operated.